Conventionally, a screw tightening device provided with various functions which can appropriately, smoothly and rapidly achieve the screw tightening operation is proposed and put into practice as a screw tightening device for performing the screw tightening operation by rotating/driving a driver bit by driving means such as an electric motor and the like.
For example, in the electric driver configured such that the driver bit is coupled to a drive output shaft of the electric motor via a clutch mechanism to perform screw tightening operations, when a screw is to be tightened to a screw hole provided in a required screw mounting target by the screw tightening device, a predetermined screw tightening torque value is reached in a state in which the screw is not completely screwed, and the clutch mechanism is operated so as to complete the screw tightening operation.
The applicant previously developed a screw tightening device which can detect defective screw tightening such as galling of the screw, screw lifting and the like with respect to the screw hole appropriately and reliably at a low cost by an easy and relatively simple configuration and filed a patent application (see Patent Document 1).
That is, the screw tightening device described in Patent Document 1 is configured such that, in a screw tightening device configured such that a screw tightening operation is performed by rotating/driving a rotary tool such as a driver bit and the like by driving means such as an electric motor and the like and a load torque generated in the rotary tool is detected with completion of screw tightening to a required screw mounting target, and when the load torque reaches a torque value set in advance, rotation/driving of the rotary tool is subjected to stop control, configured such that (1) rotation amount detecting means is provided on the rotary tool or driving means for detecting a rotation amount based on a rotation number or rotation time associated with rotation/driving of the rotary tool; and (2) at a point of time when a tip end of a screw shaft of a screw in which a screw head part is fitted with a tip end part of the rotary tool is positioned at and brought into contact with a screw hole of the screw mounting target, a screw-tightening reference time (t1) is set by screw-tightening reference time setting means by pressing the rotary tool in an axial direction.
Then, it is configured such that (3) after the screw-tightening reference time is set by the screw-tightening reference time setting means, screw-tightening start time (t2) is set by the screw-tightening start time setting means by starting the driving means of the rotary tool; (4) screw-tightening completion time (t3) when the screw positioned at and brought into contact with the screw hole by rotation/driving of the rotary tool is rotated, and the load torque generated in the rotary tool reaches the torque value set in advance is detected by screw-tightening completion time detecting means; and (5) after the screw-tightening reference time (t1) is set by the screw-tightening reference time setting means, from the screw-tightening start time (t2) when the driving means of the rotary tool is started by the screw-tightening start time setting means to the screw-tightening completion time (t3) detected by the screw-tightening completion time detecting means, it is determined whether or not a rotation amount of the rotary tool detected by the rotation amount detecting means is within a permissible range as compared with a reference value set in advance, and acceptability of the screw tightening is determined by screw-tightening acceptability determining means.
Moreover, in the screw tightening device described in Patent Document 1, it is disclosed that, a screw-tightening completion time detection signal when the clutch is operated by a torque setting clutch mechanism provided at a shaft coupling portion between a drive shaft of the driving means for rotating/driving the rotary tool and the rotary tool when a torque set value set in advance is reached is used or a screw-tightening completion time detection signal when a load current value set in advance is reached by, load current detecting means for detecting a load current of the electric motor for rotating/driving the rotary tool is used as the screw-tightening completion-time detecting means, respectively.
Moreover, the applicant developed a screw tightening device provided with an automatic stop device which can perform torque control by automatically stopping driving of the electric driver driving the rotary tool by detecting a load current of the electric motor without providing the above-described clutch mechanism in a prior art (see Patent Document 2), which was granted a patent.
That is, the screw tightening device provided with the automatic stop device described in Patent Document 2 is configured such that, if the electric motor is rotated/driven in a certain state, in the screw tightening operation, a load current when the drive shaft of the electric motor is rotated/driven becomes an overload current by a reaction force in proportion with a screw tightening torque value imparted to the drive shaft and thus, when the overload current in proportion with the screw tightening torque value set in advance reaches a required value, this state is detected, and a power supply of the electric motor is shut off so as to automatically stop the electric driver.